Membrane fouling is a problem encountered in membrane filtration processes, and it is a major factor in determining their practical application in water and wastewater treatment and desalination in terms of technology and economics. Membrane fouling includes inorganic fouling/scaling, particulate/colloidal fouling, and organic fouling with dead organic matter or live microorganisms, which would typically form a biofilm. Fouling due to organic and inorganic components and microorganisms can occur simultaneously, and these components may interact in terms of mechanism.
A well-known means for cleaning filtration membranes is “cleaning-in-place” (CIP). A typical CIP cycle will often include the use of chemicals for high pH (e.g., NaOH), low pH (e.g., citric acid or nitric acid), water and biocides for disinfection, e.g., bleach. A typical CIP cycle for cleaning membranes used in the dairy industry consists of many steps which often include:
Flush/Pre-rinse with clean water flux, which may be heated up to 60-80° C. and recirculated for a period of time.
Circulation of caustic soda solution (NaOH) at a temperature about 60-80° C. for a period of time.
Intermediate rinsing with clean water flux.
Circulation of acidic solution for a period of time.
Final rinse with clean water flux.
Final air blow.
Similar processes exist for membrane cleaning in other business segment, and the individual steps can be applied in different order dependent on the nature of the membrane fouling. A typical CIP cycle for cleaning membranes used in the enzyme manufacturing industry includes water flush, a cold and hot caustic rinse, and nitric acid rinse.
The use of harsh chemicals in CIP is undesirable and poses a problem to the environment. In the past years CIP methods including the use of enzymes have been developed.
International patent application WO 97/02753 concerns a solution comprising a protease and a lipase for cleaning-in-place. The solution has been found effective in cleaning process equipment containing residues of milk or burnt milk.
Compared to a typical CIP procedure, there are several benefits of improved membrane cleaning by use of DNase enzymes, such as, saving power to pump liquid through less fouled membranes, reducing use of harsh and hazardous chemicals, reducing cost for neutralising acids and bases and discharge after use, extension of membrane lifetime due to milder cleaning conditions and removal of fouling not removed by chemical or physical means.